Fused anti-soiling and anti-reflective coatings

ABSTRACT

A method for applying a coating to a surface includes the step of providing a reaction mixture comprising a silicon alkoxide and an alcohol. A reaction limiting amount of water is added. The silicon alkoxides and water are allowed to react to form silica precursor particles during an initial reaction period. A coating precursor composition is prepared by adding an acid soluble in the alcohol to the reaction mixture during a second reaction period after the initial reaction period. The precursor silica particles grow to form silica nanofeatures having a major dimension that is larger than a major dimension of the silica precursor particles. The coating precursor composition is applied to a surface, and the alcohol and water are allowed to evaporate and the silica nanofeatures to adhere to the surface and form a nanostructured layer on the surface. A coating precursor composition and a coated article are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional patent application of U.S. patentapplication Ser. No. 16/203,127 filed on Nov. 28, 2018, the entiredisclosure of which incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in this invention.

FIELD OF THE INVENTION

The present invention is directed to nanotextured silica surfaces, andmore particularly to methods of making nanotextured silica surfaces.

BACKGROUND OF THE INVENTION

High-performance solar glass and applicable coatings systems are in highdemand with the development of the global solar industry includingphotovoltaic and concentrated solar power electricity generation. Solarpower systems are usually exposed to harsh environments, arid orsemi-arid desert, where there is abundant sunlight, and where thesesystems can be easily contaminated with airborne sand and dust. Suchsystems require periodic cleaning of the exposed surfaces of the solardevice to efficiently generate solar power. Therefore, development ofhigh performance coatings having excellent optical clarity with minimalreflectance, anti-soiling properties, and durability to outdoorenvironmental conditions and cleaning processes, is necessary to achievehigh efficiency on solar energy systems as well as reductions inmaintenance and operating cost.

Silica thin films have been widely used in coating applications due toits unique features of high transmission, low refractivity index, gooddurability and environmental resistance. Silica thin films can beprepared by the sol-gel process such as the Stöber process usingdifferent coating methods, such as spin, dip, or drawdown coating. Thebenefit of the sol-gel method is that the structure of the resultingsilica thin film can be easily tailored with the reaction condition andits simple and low-cost processing. In the case of a base catalyzedsilica sol-gel reaction, colloidal gels are formed and the resultingfilms have high roughness. These films can be easily removed from thesubstrates due to weak interaction between particles and to thesubstrate. With an acid catalyst, linear siloxane polymers are formed inthe sol, resulting in a dense and highly cross-linked network structurewith smooth surface. A base/acid two catalyst system has been proposedand this process can achieve multiple properties that cannot be possiblewith each individual catalyst, however, the two catalyst processrequires at least two steps which makes the processing complicated andimpractical for many applications.

SUMMARY OF THE INVENTION

A method for applying a coating to a surface includes the step ofproviding a reaction mixture comprising a silicon alkoxide and analcohol. The silicon alkoxide is at least partially soluble in thealcohol. A reaction limiting amount of water is added to the siliconalkoxide and alcohol. The water is at least partially miscible with thealcohol. The silicon alkoxides and water are allowed to react to formsilica precursor particles during an initial reaction period. A coatingprecursor composition is prepared by adding an acid soluble in thealcohol to the reaction mixture during a second reaction period afterthe initial reaction period. The precursor silica particles will grow toform silica nanofeatures having a major dimension that is larger than amajor dimension of the silica precursor particles. The coating precursorcomposition is applied to a surface. The alcohol and water are allowedto evaporate and the silica nanofeatures adhere to the surface and forma nanostructured layer on the surface.

The silicon alkoxide can be at least one selected from the groupconsisting of tetraethyl orthosilicate (TEOS) and [ortetramethylorthosilicate (TMOS). The alcohol can be ethanol. The acidcan be at least one selected from the group consisting of hydrochloricacid and sulfuric acid.

The nanofeatures can include at least one selected from the groupconsisting of silica spheres and silica rods.

The method can include, after the step of applying the coating precursorcomposition to a surface, applying a heat treatment to the coatingprecursor composition. The heat treatment can have a temperature of 20°C. to 500° C. The applying step can be performed at ambient temperatureand pressure. The coating can be applied at a thickness of between0.01-1 μm. The method can further comprise the step of applying ahydrophobic composition to the nanostructured layer.

The reaction mixture can comprise 1 silicon alkoxide, 2-10 alcohol, and1-4 water, by molar ratios. The acid can be added to the reactionmixture 15 min after the water is added to the silicon alkoxides andalcohol. The method can include the step of, after the acid is added,waiting at least 15 min and then cooling the mixture to roomtemperature. The initial reaction period can be from 1-30 min. Thesecond reaction period can be from 1-30 min.

The nanofeatures can have a diameter of 10-500 nm. The nanofeatures canhave a width of 200 nm-1500 nm. The nanofeatures can have a height of10-100 nm. The nanofeatures can have a spacing of 1-500 nm.

The silicon alkoxide/alcohol molar ratio can be 1 silicon alkoxide to2-6 alcohol to obtain silica spheres. The silicon alkoxide/alcohol molarratio can be 1 silicon alkoxide to 7-10 alcohol to obtain silica rods.The silicon alkoxide/alcohol molar ratio can be from 1:4 to 1:8. Thesilicon alkoxide/alcohol molar ratio can be 1:8.

A coating precursor composition can include a sol-gel comprising water,alcohol, and silica nanofeatures. The silica nanofeatures can include atleast one selected from the group consisting of silica spheres andsilica rods, and are the reaction product of silicon alkoxide and water.

A coated article can include a silica substrate and a continuous coatingof silica nanofeatures. The coating can have a thickness of between 0.01μm-1 μm. The nanofeatures can have a diameter of 10 nm-500 nm, a widthof 200 nm-1500 nm, a height of 10 nm-100 nm, and a spacing of 1 nm-500nm. The silica nanofeatures can be silica rods.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the invention is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 is a schematic depiction of a process according to the invention.

FIGS. 2A-2D are SEM images of FIG. 2A is control silica thin film, andnanotextured silica thin film coatings prepared according to theinvention at molar ratios of TEOS to EtOH of FIG. 2B 1:2; FIG. 2C 1:4;and FIG. 2D 1:8.

FIGS. 3A-3C and 3F-3H are AFM topology images of FIGS. 3A, 3F a controlsilica thin film, and nanotextured silica thin film coating preparedaccording to the invention at a molar ratio of TEOS to EtOH of: FIGS.3B, 3G 1:2; FIGS. 3C, 3H 1:4.

FIG. 4 is a table and plot of adhesion force measurements on thesurfaces of silica thin film coatings on Si(100) substrate prepared atcontrol, and at a molar ratio of TEOS to EtOH of 1:2 and 1:4.Measurements were performed at 10 different locations.

FIGS. 5A-5C are SEM images of nanotextured silica thin film coatingsprepared at 1:2 molar ratio of TEOS to EtOH with a scale of: FIG. 5A 1μm; FIG. 5B 200 nm; and FIG. 5C 1 μm and at a defect area.

FIGS. 6A-6C are SEM images of nanotextured silica thin film coatingprepared at 1:4 molar ratio of TEOS to EtOH with a scale of d FIG. 6A 1μm; FIG. 6B 200 nm; and FIG. 6C 1 μm and at a defect area.

FIGS. 7A-7C are SEM images of nanotextured silica thin film coatingprepared at 1:8 molar ratio of TEOS to EtOH with a scale of FIG. 7A 3μm; FIG. 7B 1 μm; and FIG. 7C 3 μm and at a defect area.

FIGS. 8A-8C are silica sol prepared without acid catalyst at molarratios of TEOS to EtOH of: FIG. 8A 1:2; FIG. 8B 1:4; and FIG. 8C 1:8.

FIGS. 9A-9B are plots of transmittance (%) vs wavelength (nm) for ananotextured silica thin film coated on solar glasses FIG. 9A before andFIG. 9B after soiling test. The solid lines correspond to the averagevalues and the dotted lines correspond to the standard deviation of themeasured values. Spectra were obtained in 10 different locations.

FIGS. 10A-10D are 3D images of pillar structures according to atomicforce microscope (AFM) examination where FIG. 10A is an image size ofapproximately 10×10 μm with a height of 50 nm; FIG. 10B is approximately5×5 μm; FIG. 10C is approximately 2×2 μm; and FIG. 10D is approximately1×1 μm.

FIGS. 11A-11D are SEM figures of coatings according to the inventionmolar ratios of TEOS:EtOH of 1:2 FIG. 11A at a scale of 1 μm; TEOS:EtOHof 1:4 in FIG. 11B at a scale of 1 μm; TEOS:EtOH 1:4 in FIG. 11C at ascale of 1 μm; and TEOS:EtOH 1:4 in FIG. 11D at a scale of 2 μm. Theimages were taken at defect sites to illustrate the integrity of thesurrounding coating.

FIGS. 12A and 12B are comparative image SEM images of coatingscomprising spray coated nanoparticles at a scale of 1 μm and 200 nm.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides an efficient method to fabricate transparentnanotextured silica thin film coatings exhibiting anti-soiling behavioras well as enhanced durability. An acid catalyst is used to providehighly cross-linked network structures to accomplish high durability onsilica thin film. The roughness and morphologies of silica thin filmsare tailored through a simply controlled growth pathway by adding waterand acid catalyst sequentially rather than changing any variables. Theroughness of silica thin film is controllable, making it possible toprepare uniform and scalable coatings on solar glass and other surfaces.The prepared nanotextured silica thin films provide enhancedanti-soiling properties without compromising the optical properties, andwhile provide sufficient mechanical properties.

A method for applying a coating to a surface is shown schematically inFIG. 1 and includes the step of providing a reaction mixture comprisinga silicon alkoxide and an alcohol. The silicon alkoxide is at leastpartially soluble in the alcohol. A reaction limiting amount of water isadded to the silicon alkoxide and alcohol. The water is at leastpartially miscible with the alcohol. The silicon alkoxides and water areallowed to react to form silica precursor particles during an initialreaction period. A coating precursor composition is prepared by addingan acid soluble in the alcohol to the reaction mixture during a secondreaction period after the initial reaction period. The precursor silicaparticles will grow to form silica nanofeatures having a major dimensionthat is larger than a major dimension of the silica precursor particles.The coating precursor composition is applied to a surface, and thealcohol and water are allowed to evaporate and the silica nanofeaturesadhere to the surface and form a nanostructured layer on the surface.

The silicon alkoxide can be at least one selected from the groupconsisting of tetraethyl orthosilicate (TEOS) and [ortetramethylorthosilicate (TMOS). Other silicon alkoxides are possible.

The alcohol can be ethanol. Other alcohols are possible so long as thesilicon alkoxides and water are partially miscible in the alcohol, andthe alcohol does not participate in the reaction.

The acid can be at least one selected from the group consisting ofhydrochloric acid and sulfuric acid. Other acids are possible so long asthe acid will catalyze the formation of the nanofeatures.

The nanofeatures can include at least one selected from the groupconsisting of silica spheres and silica rods. The size and geometry ofthe spheres and rods can be controlled according to the process of theinvention.

A heat treatment can be applied to the coating precursor compositionafter the step of applying the coating precursor composition to asurface. The heat treatment can have a temperature of from 20° C. to500° C. The heat treatment will assist in evaporation of the water andalcohol.

The applying to the surface step can be performed at ambient temperatureand pressure. The application can be made by conventional methods.

The reaction mixture can vary in the relative proportions of siliconalkoxide, alcohol and water. The reaction mixture can include 1 siliconalkoxide, 2-10 alcohol, and 1-4 water, by molar ratios. Other ratios arepossible. The molar proportion of alcohol for each mole of siliconalkoxide can be 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, 4, 4.25, 4.5,4.75, 5, 5.25, 5.5, 5.75, 6, 6.25, 6.5, 6.75, 7, 7.25, 7.5, 7.75, 8,8.25, 8.5, 8.75, 9, 9.25, 9.5, 9.75 or 10, or within a range of any highand low value selected from these values. The molar proportion of waterfor each mole of silicon alkoxide can be 1, 1.25, 1.5, 1.75, 2, 2.25,2.5, 2.75, 3, 3.25, 3.5, 3.75 or 4, or within a range of any high andlow value selected from these values.

The ratio of silicon alkoxide to alcohol can be used to control thegeometry and size of the resulting nanofeatures. In general, the higherthis ratio is the smaller and spherical will be the nanofeatures, whileat lower ratios the features will be larger and more rod or pillar-likein geometry. More rod-like features will impart a greater amount oftexture and hence hydrophobicity to the resulting coating. For example,the silicon alkoxide/alcohol molar ratio can be 1 silicon alkoxide to2-6 alcohol to obtain silica spheres. The silicon alkoxide/alcohol molarratio to obtain semi-spheres can be 1 silicon alkoxide to 2, 2.5, 3,3.5, 4, 4.5, 5, 5.5, or 6 moles alcohol, or within a range of any highor low value selected from these values. The silicon alkoxide/alcoholmolar ratio can be 1 silicon alkoxide to 7-10 alcohol to obtain silicarods. The silicon alkoxide/alcohol ratio to obtain rods can be 1 siliconalkoxide to 7, 7.5, 8, 8.5, 9, 9.5, or 10 moles alcohol, or within arange of any high and low value selected from these values. It has beenfound that silicon alkoxide to alcohol ratios of from 1:4 to 1:8 providemore texture than surfaces prepared with a 1:2 ratio. A ratio of 1:8 hasbeen found to produce rods or pillars of generally cylindrical shape.

The acid can be added to the reaction mixture after a period of time haspassed in order to permit the initial reaction without acid to proceed.For example, the acid can be added 15 min after the reaction-limitingamount of water is added to the silicon alkoxides and alcohol. After theacid is added, the process can include waiting at least 15 min and thencooling the mixture to room temperature. The acid can be added dropwiseor by other dosing protocols.

The initial reaction period during which the reaction-limiting amount ofwater is allowed to react with the silicon alkoxide can vary. Theinitial reaction period can be from 1-30 min. The initial reactionperiod can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30 min, or within arange of any high and low value selected from these values.

The initial reaction during which the reaction-limiting amount of wateris allowed to react with silicon alkoxide can occur at an elevatedtemperature. The elevated temperature can be 25, 30, 35, 40, 45, 50, 55,60, 65, 70, 75, 80, 85, 90, 95, or 100° C., or within a range of anyhigh value and low value selected from these values. The temperature atwhich the initial reaction is conducted can be 60° C.

The second reaction period during which the reaction is acid catalyzedcan vary. For example, the second reaction period can be from 1-30 min.The second reaction period can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or30 min, or within a range of any high and low value selected from thesevalues.

The thickness of the coating that is applied can vary. The coating canbe applied at a thickness of between 0.01-1 μm. The coating can beapplied at a thickness of 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07,0.08, 0.09, 0.10, 0.20, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1 μm, orwithin a range of any high and low value selected from these values.

The nanofeatures that are formed according to the invention can havedifferent shapes and sizes. The nanofeatures can have a diameter of10-500 nm. The nanofeatures can have a diameter of 10, 20, 30, 40, 50,60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200,210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340,350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480,490, or 500 nm, or within a range of any high and low value selectedfrom these values.

The nanofeatures can have a width of 200 nm-1500 nm. The nanofeaturescan have a width of 200, 225, 250, 275, 300, 325, 350, 375, 400, 425,450, 475, 500, 525, 550, 575, 600, 625, 650, 675, 700, 725, 750, 775,800, 825, 850, 875, 900, 925, 950, 975, 1000, 1025, 1050, 1075, 1100,1125, 1150, 1175, 1200, 1225, 1250, 1275, 1300, 1325, 1350, 1375, 1400,1425, 1450, 1475 or 1500 nm, or within a range of any high and low valueselected from these values.

The nanofeatures can have a height of 10-100 nm. The nanofeatures canhave a height of 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75,80, 85, 90, 95 or 100 nm, or within a range of any high and low valueselected from these values.

The nanofeatures can have a spacing between nanofeatures of 1-500 nm.The nanofeatures can have a spacing of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170,180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310,320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450,460, 470, 480, 490, or 500 nm, or within a range of any high and lowvalue selected from these values.

The method can further include the step of applying a hydrophobiccomposition to the nanostructured layer. The hydrophobic composition canbe selected from many different materials that will adhere to thecoating and provide hydrophobicity, such as hydrophobic silanes based onfluorocarbons, and alkyl groups. The combination of the textured coatingwith nanofeatures formed according to the invention, and a hydrophobiclayer can impart superhydrophobic properties to surface.

A coating precursor composition includes a sol-gel comprising water,alcohol, and silica nanofeatures. The silica nanofeatures comprise atleast one selected from the group consisting of silica spheres andsilica rods, and are the reaction product of silicon alkoxide and water.

A coated article according to the invention includes a silica substrateand a continuous coating of silica nanofeatures. The coating can have athickness of between 0.01 μm-1 μm. The nanofeatures can have a diameterof 10 nm-500 nm, a width of 200 nm-1500 nm, a height of 10 nm-100 nm,and a spacing of 1 nm-500 nm. The silica nanofeatures can be silicarods.

A variety of structures can be generated through adjustment of the molarratios of components in sol-gel reaction system. Due to the micro-phaseseparation phenomena in the silica sol solution, the structure of theresulting silica thin film was significantly different depending on theamount of EtOH that was present in the reaction mixture. TEOS isinsoluble in water and the hydrolysis occurs at the interface ofTEOS/water. The interaction between water and TEOS molecules iscontrolled by amount of EtOH present, which acts as a co-solvent inwhich both water and TEOS are soluble.

Example

The silica sol-gel precursor was prepared by mixing tetraethylorthosilicate (TEOS), ethanol (EtOH), water (H₂O) and hydrochloric acid(HCl) with different molar ratios. First, TEOS was dissolved indifferent amounts of EtOH and stirred at room temperature for 10 min.While stirring, H₂O was added drop wise to the solution and it washeated to 60° C. for 15 min. TEOS was partially hydrolyzed with adeficient amount of water in solution. After 15 min, HCl was added intothe solution as a catalyst, so that hydrolysis and condensation reactioncould proceed further for another 15 min, and the solution was thencooled to room temperature for 30 min. The molar ratios of TEOS to EtOHin the precursor were varied in each sample, which are designated in1:2, 1:4, 1:8, and the TEOS:H₂O:HCl molar ratio was kept constant at1:2:0.01 in all of the solutions. For comparison, silica sol-gelprecursor with simultaneous mixing of materials was also prepared as acontrolled trial, at 1:2 molar ratio of TEOS to EtOH.

The prepared silica sols were then applied on clean solar glasses (lowiron glass) or solar mirror substrates using drawdown coating techniquewhich is a representation of the curtain coating approach in the coatingindustry. An automated drawdown coater (model DP-8301, GARDCO, PompanoBeach Fla.) with 3 wire-wound Meyer rod was used for coating at a speedrate of 1 in/sec. The coated substrates were then dried in ambientcondition and formed silica gel film with various nanotexture.

Field emission scanning electron microscopy (SEM) and atomic forcemicroscopy (AFM) were used to characterize morphology and topography ofcoated surface. AFM probe with spherical silica particle (15 μmdiameter, NanoAndMore, Watsonville Calif., USA) was used to quantify theadhesion force and energy dissipation on the coated surface. Thespherical silica particle in AFM probe is used as a model for soilingeffect of silica-based dust on the glass surface. For AFM measurement,the coatings were applied on Si(100) substrate. Anti-soiling propertieswere investigated by falling sand testing according to ASTM D968 usingISO 12103-1 A4 coarse test dust (average particle diameter of 55 μm).Then 2 g of the test dust was introduced to the test apparatus (120 cmlong, 7.6 cm inside diameter) and distributed on the test surface underthe apparatus. After dust accumulation, air-brushing with thesqueezed-bulb dust blower (air volume=˜32-40 ml) was applied to removethe loose dust. To evaluate anti-soiling properties, the opticalproperties (transparency on solar glass) of the coatings werecharacterized before and after the soiling testing using UV-visspectroscopy. The mechanical properties of silica thin film wereevaluated with nano-indentation and the tape test (ASTM D3359).

Generally, fibrous structures are formed when the sol gel process isperformed under the acidic conditions. FIG. 2 are SEM images of (A)controlled silica thin film, and nanotextured silica thin film coatingsprepared at FIG. 2(B) 1:2, FIG. 2(C) 1:4, and FIG. 2(D) 1:8 molar ratiosof TEOS to EtOH. Therefore, films made from a sol made in acidicconditions result in smooth and flat films as shown in SEM image of FIG.2(A) where HCl is added in the beginning along with other reagents. Theprocess of the invention adds the HCl catalyst only after an initialreaction period, for example, after 15 minutes, and allows the sol-gelreaction to proceed at 60° C. When the reaction is allowed to proceed inthe absence of the acidic catalyst, but at high temperature, for examplehigher than 50° C., though the reaction is slow some nucleation occursbecause of the high temperature. This initial nucleation results in theformation of nanoscale silica particle like structures. When the HClcatalyst is added to the reaction mixture, it results inbimodal—particulate and fibrous—reaction growth. Particulate structuresare formed by further deposition of silica on the nucleates formedbefore addition of HCl, and fibrous structures start growing afteradding HCl. Fibrous structures result as TEOS is not completely consumedwith deficient amount of water (1:2 molar ratio of TEOS to H₂O) duringthe 15 min time interval, and free TEOS precursor in the solutionfollows typical silica chain growth mechanism, growing flexible andlinear structure siloxane polymers, with the acid catalyst. When thisreaction mixture is used for making the film, the film shows particulatefeatures over the coated substrate as shown in FIG. 2 and FIG. 3. FIG. 3shows AFM topology images of controlled silica thin film in FIG. 3(A,F), and nanotextured silica thin film coatings prepared at FIG.3(B,G) 1:2, and FIG. 3(C,H) 1:4, molar ratios of TEOS to EtOH.

At low concentrations of EtOH, such as a 1:2 molar ratio of TEOS toEtOH, the solution has very low homogeneity. The interaction betweenTEOS and water molecules is restricted, which results in slowedhydrolysis, and thus slow growth of silica particles. Therefore, atlower EtOH concentrations only small silica nanoparticles are formed asshown in FIG. 2(B) and FIG. 3(B). An increase in the EtOH concentrationresults in the enhanced solubility of TEOs in the reaction mixture, andthus increased interaction between water and TEOS molecules, whichresults in an enhanced hydrolysis of TEOS molecules. Due to theincreased hydrolysis rate, more hydrolysed TEOS molecules are availableto deposit on initially nucleated nanoparticles, which results in largerparticles a shown in FIG. 2(C) and FIG. 3(C) where TEOS to EtOH ratio is1:4. At very high EtOH concentrations, such as 1:8 molar ratio of TEOSto EtOH, spheroids or oblong particles with high polydispersity wereobserved in FIG. 2(D). The high homogeneity of the solution gives riseto spinodal decomposition in sol-gel system, and different nanotextureswere formed.

Since the coated surface can be easily exposed to any damage (whilehandling or cleaning) and environment, the coating should have goodmechanical properties and high adhesion to the substrate.Nano-indentation and the tape test were used to evaluated mechanicalproperties of silica thin film. Nano indentation test give quantitativeinformation on elastic modulus (E) and hardness (H) of the preparedsilica thin film.

A transparent 3M Scotch tape was applied onto the silica thin film andthen peeled it off quickly. No change was found on silica thin filmindicating a strong adhesion to solar glass substrate.

The adhesion force was measured on the surfaces of silica thin filmcoating on Si(100) substrate prepared at control, 1:2 and 1:4 silicasol-gel reaction condition using AFM. The silica thin film prepared at1:8 was excluded for the measurement. Due to their large surfacefeature, the adhesion force cannot be determined accurately, and it isnot reasonable to measure adhesion on large features. FIG. 4 depicts theresults of adhesion force measurements on the surfaces of silica thinfilm coating on Si(100) substrate prepared at control, 1:2 and 1:4silica sol-gel reaction condition. Measurements were performed on 10different area. As shown in FIG. 4, the control sample with flat andsmooth surface exhibit large adhesion force, 164 nN. However,dramatically decreased was observed on prepared nanotextured silica thinfilms. Compared to the control sample, they exhibit ˜4-fold and 15-foldless interaction between the silica sphere on AFM probe to the coatedsurface, respectively. This decreased adhesive force account to thenanotexture of the prepared silica thin film. According to the Rump andthe Rabinovich adhesion force models, the adhesion forces between asubstrate and a particle is strongly dependent on root mean square (rms)surface roughness and the distance between the surface asperities (1),Equation 1. The required energy dissipation for separation is ˜15 timesand ˜120 times smaller compared to the smooth surface of control sample.

Equation 1. The adhesion force between an adhering particle and asurface with nanoscale roughness is;

$\begin{matrix}{F_{ad} = {\frac{AR}{6H_{0}^{2}}\left\lbrack {\frac{1}{1 + \frac{32Rk_{1}{rms}}{\lambda^{2}}} + \frac{1}{\left( {1 + \frac{k_{1}{rms}}{H_{0}}} \right)^{2}}} \right\rbrack}} & (1)\end{matrix}$

where A is the Hamaker constant, R is the radius of the adheringparticle, Ho is the distance between the particle and the surface (˜0.3nm, when particle is in contact with the surface), rms is root meansquare surface roughness, 1 is the distance between the surfaceasperities and k₁ is a constant (1.817). Equation 1 accounts only forvan der Walls attraction between sand particles and surface ofsubstrate.

Acidic conditions further stimulate the cross-linking of the particleswith the fibrous structures, resulting in continued and durablenanotextured silica thin film. Also shown in FIGS. 5-7 are SEM imagestaken of the coatings and at the defect area at the edge of substratewhere the coating was not uniform. FIGS. 5-7 show the formation of acontinuous silica film structure.

FIG. 5 shows SEM images of nanotextured silica thin film coatingprepared at 1:2 molar ratio of TEOS to EtOH with different magnificationand at the defect area. FIG. 5 is SEM images of nanotextured silica thinfilm coating prepared at 1:2 molar ratio of TEOS to EtOH with a scaleof: FIG. 5(A) 1 μm; FIG. 5(B) 200 nm; and FIG. 5(C) 1 μm and at a defectarea to show the continuous nature of the surrounding coating.

FIG. 6 is SEM images of nanotextured silica thin film coating preparedat 1:4 molar ratio of TEOS to EtOH with a scale of d FIG. 6(A) 1 μm;

FIG. 6(B) 200 nm; and FIG. 6(C) 1 μm and at a defect area to show thecontinuous nature of the surrounding coating.

FIG. 7 is SEM images of nanotextured silica thin film coating preparedat 1:8 molar ratio of TEOS to EtOH with a scale of FIG. 7(A) 3 μm; FIG.7(B) 1 μm; and FIG. 7(C) 3 μm and at a defect area to show thecontinuous nature of the surrounding coating.

FIG. 8 is silica sol prepared without acid catalyst at a molar ratio ofTEOS to EtOH of: FIG. 8(A) shows a coating prepared at a ratio of 1:2.FIG. 8(B) shows a coating prepared at a ratio of 1:4. FIG. 8(C) shows acoating prepared at a ratio of 1:8. Without acid catalyst, preparedsilica sol did not adhere onto the substrate and there was not texturedcoating film formation.

The nanostructured features are fused together and form a continuousfilm. The developed coatings can be fused onto the surface of glass orother substrates. The coating is fused on the desired surface and isvery durable. It can be applied using draw down or spray-on applicationof a solution. After the curing of the applied solution, a fused layeris formed.

Observation of before and after soiling tests on solar coated anduncoated glass and solar mirror were performed. The 1:4 molar ratio ofTEOS:EtOH silica sol was applied to the coated side. The uncoated sideshowed significantly more soiling than the coated side.

Sand falling testing was performed on half-coated solar mirrors with thesol-gel based anti-soiling coating. The coatings were fluorocarbon free.The coated side showed significantly more soiling than did the coatedside. After rinsing with residual water the coated side showed greaterresponse to sand removal by the water than did the uncoated side.

To further evaluate the anti-soiling property of the coatings, theoptical transmittance measurements in the ultraviolet-visible spectralregion (200-1100 nm) were performed before and after soiling test at 10different areas. The decrease in transmittance is strongly correlated tothe degree of soiling. The same coating was applied on solar glass andthe transmittance was measured before and after the soiling experiments.The coated glass showed greater resistance to soiling than did theuncoated glass. The transmittance spectra is shown in FIG. 9. Theaverage decrease in the transmittance of the soiled uncoated solar glassis 20%. FIG. 9 is the transmittance spectra of nanotextured silica thinfilm coated on solar glasses (a) before and (b) after soiling test. Thesolid lines correspond to the average values and the dotted linescorrespond to the standard deviation of the measured values. Spectrawere obtained in 10 different areas. FIG. 9 shows that the maximumtransmittance of the solar glass is reached as high as 92.4% in thevisible range (380-760 nm) while the maximum transmittance of the silicathin film coated solar glass is slightly increased to 93.1-94.3% due toits anti-reflective effect of silica thin film. After soiling test, theuncoated solar glass loss approximately 23% of transmittance with verylarge variation, ±25.3%. due to non-uniform soiling on the substrate.However, it is noted that no significant difference was observed beforeand after soiling test on coated solar glassed. They maintain nearly thesame transmittance with less than 1% variation which indicate highuniformity of the coating. The optical images of before and aftersoiling test on solar glass and mirror clearly show the huge differencesoiling result on coated and uncoated solar glass and mirror, (One halfwas coated, and the other half was uncoated to allow a comparison).

The invention produced very good nanotexture in the form of pillars.FIG. 10 shows 3D images of pillar structures according to atomic forcemicroscope (AFM). The images show well-defined pillars and significantnanotexture formed in a continuous coating.

FIG. 11 shows SEM figures of coatings according to the invention molarratios of TEOS:EtOH of 1:2, FIG. 11(A); TEOS:EtOH of 1:4 in FIG. 11(B)at a scale of 1 μm; TEOS:EtOH 1:4 in FIG. 11(C) at a scale of 1 μm; andTEOS:EtOH 1:4 in FIG. 11(D) at a scale of 2 μm. The images were taken atdefect sites to illustrate the integrity of the surrounding coating.

FIG. 12 is a comparative image SEM images of coatings comprising spraycoated nanoparticles at a scale of FIG. 12(A) 1 μm and FIG. 12(B) 200nm.

Advantages over existing anti-soiling coatings include: this inventionis one-step application method of a solution; durability—the coating isfused on the glass surface and is not an add-on film; there are noorganic components and no UV degradation; there is no chemical etchingor sputtering; and the invention is low cost, scalable, easy to retrofitand re-apply, and environmentally friendly.

The invention provides a one-step approach to fabricated transparentnanotextured silica thin film that can be controlled by manipulatingsilica sol preparation method. The prepared coating exhibited rough andcontinuous film structure, and the nanotextured surface contributed tothe enhanced anti-soiling efficiency. There are general correlations onreaction condition to nanotextures of silica thin film. The adhesionforce measurements demonstrated low interaction between the coatedsurface to silica spheres on AFM probe, and sand falling testdemonstrated a high anti-soiling property to test dust. The continuousphase of the prepared silica thin film allows high durability andadhesion to the glass substrate. The coatings of the invention provideperformance in anti-soiling, optical, and mechanical properties, and canbe used in solar energy industry and in other industries such as displaydevices, or high-end windows to reduce maintenance costs and improve theenergy efficiency, and in other applications.

This invention can be embodied in other forms without departing from thespirit or essential attributes thereof. Accordingly, reference shouldalso be made to the following claims to determine the scope of theinvention.

We claim:
 1. A coating precursor composition, comprising a sol-gelcomprising water, alcohol, and silica nanofeatures, the silicananofeatures comprising at least one selected from the group consistingof silica spheres and silica rods, and being the reaction product ofsilicon alkoxide and water.
 2. A coated article comprising a silicasubstrate and a continuous coating of silica nanofeatures, the coatinghaving a thickness of between 0.01 μm-1 μm, the nanofeatures having adiameter of 10 nm-500 nm, a width of 200 nm-1500 nm, a height of 10nm-100 nm, and a spacing of 1 nm-500 nm.
 3. The coated article of claim2, wherein the silica nanofeatures are silica rods.